Graphs showing sensitivity of average annual recharge and irrigation pumpage simulated by the SOil-WATer-Balance (SOWAT) model to changes in (A) irrigation efficiencies, (B) initial soil moisture, (C) minimum soil-moisture requirement, (D) effective precipitation, and (E) evapotranspiration ..

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod/.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. The authors thank James Cannia (formerly USGS) and Dennis Strauch (Pathfinder Irrigation District) for assistance with identifying potential intentional recharge sites. The authors thank M. Fienen (USGS) for assistance with parameter estimation and R. Niswonger (USGS) for assistance with the Newton-Rhapson solver. The authors also thank P. Jones and P. Barlow (USGS) for constructive reviews of earlier versions of this report and P. Barlow for assistance with application of the Groundwater-Management Process. DatumsVertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88).Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).Altitude, as used in this report, refers to distance above the vertical datum, sea level. Simulation of Groundwater Flow and Analysis of the AbstractThe North Platte Natural Resources District (NPNRD) has been actively collecting data and studying groundwater resources because of concerns about the future availability of the highly inter-connected surface-water and ground water resources. This report, prepared by the U.S. Geological Survey in cooperation with the North Platte Natural Resources District, describes a groundwater-flow model of the North Platte River valley from Bridgeport, Nebraska, extending west to 6 miles into Wyoming. The model was built to improve the understanding of the interaction of surface-water and groundwater resources, and as an optimization tool, the model is able to analyze the effects of water-management options on the simulated stream base flow of the North Platte River. The groundwater system and related sources and sinks of water were simulated using a newton formulation of the U.S. Geological Survey modular three-dimensional groundwater model, referred to as MODFLOW-NWT, which provided an improved ability to solve nonlinear unconfined aquifer simulations with wetting and drying of cells. Using previously published aquifer-base-altitude contours in conjunction with newer test-hole and geophysical data, a new base-of-aquifer altitude map was generated because of the strong effect of the aquifer-base topography on groundwater-flow direction and magnitude. The largest inflow to groundwater is recharge originating from water leaking from canals, which is much larger than recharge originating from infiltration of pr...

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Irrigated agriculture is vital to the livelihood of communities in the Elkhorn and Loup River Basins in Nebraska, and ground water is used to irrigate most of the cropland. Concerns about the sustainability of groundwater and surfacewater resources have prompted State and regional agencies to evaluate the cumulative effects of groundwater irrigation in this area. To facilitate understanding of the effects of groundwater irrigation, a numerical computer model was developed to simulate groundwater flow and assess the effects of groundwater irrigation (including groundwater withdrawals, hereinafter referred to as pumpage, and enhanced recharge) on stream base flow. The study area covers approximately 30,800 square miles, and includes the Elkhorn River Basin upstream from Norfolk, Nebraska, and the Loup River Basin upstream from Columbus, Nebraska. The water-table aquifer consists of Quaternary-age sands and gravels and Tertiary-age silts, sands, and gravels. The simulation was constructed using one layer with 2-mile by 2-mile cell size. Simulations were constructed to represent the groundwater system before 1940 and from 1940 through 2005, and to simulate hypothetical conditions from 2006 through 2045 or 2055. The first simulation represents steady-state conditions of the system before anthropogenic effects, and then simulates the effects of early surface-water development activities and recharge of water leaking from canals during 1895 to 1940. The first simulation ends at 1940 because before that time, very little pumpage for irrigation occurred, but after that time it became increasingly commonplace. The pre-1940 simulation was calibrated against measured water levels and estimated long-term base flow, and the 1940 through 2005 simulation was calibrated against measured water-level changes and estimated long-term base flow. The calibrated 1940 through 2005 simulation was used as the basis for analyzing hypothetical